As information age progresses, flat panel display (FPD) devices having light weight, thin profile, and low power consumption have been substituted for cathode ray tube (CRT) devices. Liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, and electroluminescent display (ELD) devices are examples of the FPD devices. LCD devices have been widely used for a notebook computer, a monitor and a television because of their superiority in displaying moving images and high contrast ratio. Since the LCD device is a non-emissive type, an additional light source is required for the LCD device. For example, a backlight unit is disposed under a liquid crystal panel to supply light.
The backlight unit may be classified into a side light type and an edge light type according to a position of the light source. In the side light type backlight unit, light from the light source is refracted in a light guide plate and emitted to the liquid crystal panel. In the direct type backlight unit, a plurality of light sources are disposed directly under the liquid crystal panel and light from the plurality of light sources is emitted to the liquid crystal panel.
FIG. 1 is a cross-sectional view of a liquid crystal display device including a side light type backlight unit according to the related art. Referring to FIG. 1, a liquid crystal display (LCD) device includes a liquid crystal panel 10, a backlight unit 20, a top frame 40, a main frame 30 and a bottom frame 50. The liquid crystal panel 10 includes first and second substrates 12 and 14 facing and spaced apart from each other and a liquid crystal layer (not shown) between the first and second substrates 12 and 14. A printed circuit board (PCB) (not shown) may be connected to at least one side of the liquid crystal panel through a connection means, such as a flexible printed circuit (FPC).
The backlight unit 20 is disposed under the liquid crystal panel 10. The backlight unit 20 includes a lamp 24, a lamp guide 28, a reflective plate 22, a light guide plate 26 and a plurality of optical sheets 60. The lamp 24 is disposed along at least one side of the main frame 30, and the lamp guide 28 wraps the lamp 24 for reflection and protection. The reflective plate 22, the light guide plate 26 and the plurality of optical sheets 60 are sequentially disposed over the bottom frame 50.
FIG. 2 is a perspective view of a plurality of optical sheets of a backlight unit for a liquid crystal display device according to the related art. Referring to FIG. 2, a plurality of optical sheets 60 includes a diffusing sheet 61, a first prism sheet 63, a second prism sheet 65 and a protecting sheet 67. The first and second prism sheets 63 and 65 collimate light for improving brightness, and the protecting sheet 67 protects the second prism sheet 65. The first and second prism sheets 63 and 65 include first and second prism patterns 63a and 65a, respectively. The first prism pattern 63a is disposed perpendicular to the second prism pattern 65a. Accordingly, the first and second prism sheets 63 and 65 collimate light toward the liquid crystal panel 10 (of FIG. 1) to improve light efficiency and viewing angle.
FIG. 3 is a cross-sectional view of a prism sheet of a backlight unit for a liquid crystal display device according to the related art. Referring to FIG. 3, a prism sheet 63 having a rear surface and a front surface includes a prism pattern 63a corresponding to the front surface. Light from the backlight unit 20 enters the rear surface of the prism sheet 63 at first, second and third regions A, B and C of the rear surface of the prism sheet 63. Light entering the prism sheet 63 may have an incident angle of about 5°. The light entering the prism sheet 63 is refracted at the rear surface.
The light entering the first region A is refracted at the front surface of the prism pattern 63a and is emitted toward the liquid crystal panel 10 (shown in FIG. 1) to increase brightness. The light entering the second region B is totally reflected at the front surface of the prism pattern 63a and is emitted from the rear surface of the prism sheet 63 toward the rear of the backlight 20. The light emitted from the rear surface of the prism sheet 63 may be reflected at the reflective plate 22 (shown in FIG. 1) back to the prism sheet 63.
Thus, light loss through the second region B is minimized by recycling the light reflected back toward the rear of the backlight unit 20. However, loss of light through a side lobe of the prism sheet 63 causes deterioration of light efficiency and reduction of viewing angle. For example, the light entering the third region C is reflected and refracted at the front surface of the prism pattern 63a, but is not incident on the liquid crystal panel 10. As a result, the light entering the third region C may be scattered away from the liquid crystal panel 10 and be lost.
FIG. 4 is a graphical simulation of luminance of light passing through a prism sheet of a backlight unit of a liquid crystal display device according to the related art. Referring to FIG. 4, the light entering the first region A (shown in FIG. 3) is emitted along a front direction toward the liquid crystal panel 10 (shown in FIG. 1), while the light entering the third region C (shown in FIG. 3) is emitted along two opposite side directions (side lobe) and is not incident on the liquid crystal panel 10. As a result, the light entering the third region C is refracted away from the liquid crystal panel 10 and is lost as a light leakage toward an undesired direction at the front surface of the prism pattern 63a (shown in FIG. 3).
Since the plurality of optical sheets 60 (shown in FIG. 2) include the first and second prism sheets 63 and 65 (shown in FIG. 2) that have the first and second prism patterns 63a and 65a disposed perpendicular to each other, the light loss may be reduced so that light efficiency and viewing angle property can be improved. However, since the first and second prism sheets 63 and 65 are used for the plurality of optical sheets 60, process time increases and process efficiency decreases. Moreover, production cost increases. Furthermore, the thickness and the weight of the related art backlight unit 20 also increase.